This invention pertains to the field of automatic telecommunications devices which operate over telephone lines. More particularly, this invention pertains to the use of automatic telecommunication devices on telephone lines on which normal voice conversations can also occur, and in which the telecommunications device detects when a telephone on the same line is off-hook so that it may release the line and thus ensure that normal usage of the telephone is not affected by the presence of the device.
There are a large number of applications in which it is desirable to install automatic data collection and reporting devices in a remote unattended site. These devices typically report to a central computer system using some form of telecommunications modem. It is generally more desirable to install this equipment on a switched network telephone line rather than a permanently dedicated leased telephone line. Whenever possible, it is economically preferable to use an existing voice telephone line rather than install a special telephone line for the sole use of this equipment.
However, sharing a telephone line between normal subscriber usage and automatic machine usage can create some special problems. In particular, the presence of the automatic equipment should not impact on normal usage of the telephone line for voice communications. Therefore, the equipment must have some means of determining when the telephone line is being used for normal voice communication on one of the local telephones. The equipment must not attempt to use the telephone line if it can determine that the line is in use, and it must make the determination without interfering with existing traffic. When the equipment is using the telephone line, it must abandon the line if one of the local telephones goes off-hook. There is a serious liability issue involved in using the subscriber phone line. The automatic equipment must not prevent normal usage of the telephone in an emergency situation.
Determining when no devices are loading the telephone line is relatively straight forward, and requires making a DC voltage measurement between ring and tip of the telephone line. However, one problem is that the characteristics of the telephone line itself can change greatly, both from site to site and at a single site over a period of time. Moreover, when the automatic device is using the telephone line it must also be able to detect when one of the local telephones goes off hook. This is a far more difficult determination to make since there is a great variation in the line loading created by different types of telephones. In addition, all known prior art off-hook detectors have one or more of the following shortcomings: require earth ground as a voltage reference, do not provide total isolation from the line, respond slowly, malfunction in the presence of noise, generate noise which can interfere with data transmission, require interaction between local and remote locations to function, or are costly and/or bulky.